Traffic heading into San Francisco on the Bay Bridge was backed up Monday morning due to a four-vehicle crash involving an AC Transit bus and a Chevy Avalanche, according to the California Highway Patrol.

The accident happened just after 6 a.m. and has blocked two lanes of westbound Interstate Highway 80 on the eastern span of the bridge.

Police and fire officials were on the scene, but it is unknown how long the two lanes of the bridge would remain blocked, a CHP officer said.